The present invention is generally directed to USB device redirection in a virtual desktop infrastructure (VDI) environment. USB device redirection generally refers to making a USB device that is connected to a client terminal accessible within a virtual desktop as if the USB device had been physically connected to the virtual desktop. In other words, when USB device redirection is implemented, a user can connect a USB device to his or her client terminal and the USB device will function as if it had been connected to the server.
FIGS. 1 and 2 and the following description will provide a general overview of how USB device redirection can be implemented in accordance with some embodiments of the present invention. In FIG. 1, a computing system 100 is depicted as including a number of client terminals 102a-102n (referenced generally herein as client(s) 102) in communication with a server 104 via a network 106. Server 104 can be configured to support a remote session (e.g., a remote desktop session) wherein a user at a client 102 can remotely access applications and data at the server 104 from the client 102. Such a connection may be established using any of several well-known techniques such as the Remote Desktop Protocol (RDP) and the Citrix® Independent Computing Architecture (ICA).
Client terminal 102 may represent a computer, a mobile phone (e.g., smart phone), a laptop computer, a thin client terminal, a personal digital assistant (PDA), a portable computing terminal, or a suitable terminal or device with a processor. Server 104 may represent a computer, a laptop computer, a computing terminal, a virtual machine (e.g., VMware® Virtual Machine), a desktop session (e.g., Microsoft Terminal Server), a published application (e.g., Microsoft Terminal Server) or a suitable terminal with a processor.
Client 102 may initiate a remote session with server 104 by sending a request for remote access and credentials (e.g., login name and password) to server 104. If server 104 accepts the credentials from client 102, then server 104 may establish a remote session, which allows a user at client 102 to access applications and data at server 104. During the remote session, server 104 sends display data to client 102 over network 106, which may include display data of a desktop and/or one or more applications running on server 104. The desktop may include, for example, icons corresponding to different applications that can be launched on server 104. The display data allows client 102 to locally display the desktop and/or applications running on server 104.
During the remote session, client 102 may send user commands (e.g., inputted via a mouse or keyboard at client 102) to server 104 over network 106. Server 104 may process the user commands from client 102 similar to user commands received from an input device that is local to server 104. For example, if the user commands include mouse movements, then server 104 may move a pointer on the desktop running on server 104 accordingly. When the display data of the desktop and/or application changes in response to the user commands, server 104 sends the updated display data to client 102. Client 102 locally displays the updated display data so that the user at client 102 can view changes at server 104 in response to the user commands Together, these aspects allow the user at client 102 to locally view and input commands to the desktop and/or application that is running remotely on server 104. From the perspective of the client side, the desktop running on server 104 may represent a virtual desktop environment.
FIG. 2 is a block diagram of a local device virtualization system 200 in accordance with embodiments of the present invention. System 200 may include client 102 in communication with server 104 over network 106 as illustrated in FIG. 1. Client 102 may include a proxy 210, a stub driver 220, and a bus driver 230. Client 102 can be connected to a device 240, as shown in FIG. 2. Server 104 may include an agent 250 and a virtual bus driver 260.
In accordance with USB device redirection techniques, while device 240 is not locally or physically connected to server 104 and is remote to server 104, device 240 appears to server 104 as if it is locally connected to server 104, as discussed further below. Thus, device 240 appears to server 104 as a virtual device 290.
By way of illustration and not limitation, device 240 may be any type of USB device including a machine-readable storage medium (e.g., flash storage device), a printer, a scanner, a camera, a facsimile machine, a phone, an audio device (e.g., a headset), a video device (e.g., a camera), a peripheral device, or other suitable device that can be connected to client 102. Device 240 may be an external device (i.e., external to client 102) or an internal device (i.e., internal to client 102).
Bus driver 230 can be configured to allow the operating system and programs of client 102 to interact with device 240. In one aspect, when device 240 is connected to client 102 (e.g., plugged into a port of client 102), bus driver 230 may detect the presence of device 240 and read information regarding device 240 (“device information”) from device 240. The device information may include features, characteristics and other information specific to device 240 such as a device descriptor (e.g., product ID, vendor ID and/or other information), a configuration descriptor, an interface descriptor, an endpoint descriptor and/or a string descriptor. Bus driver 230 may communicate with device 240 through a computer bus or other wired or wireless communications interface.
In accordance with USB device redirection techniques, device 240 may be accessed from server 104 as if the device were connected locally to server 240. Device 240 may be accessed from server 104 when client 102 is connected to server 104 through a user session running on server 104. For example, device 240 may be accessible from the desktop running on server 104 (i.e., virtual desktop environment). To enable this, bus driver 230 may be configured to load stub driver 220 as the default driver for device 240. Stub driver 220 may be configured to report the presence of device 240 to proxy 210 and to provide the device information (e.g., device descriptor) to proxy 210. Proxy 210 may be configured to report the presence of device 240, along with the device information, to agent 250 of server 104 over network 106. Thus, stub driver 220 redirects device 240 to server 104 via proxy 210.
Agent 250 may be configured to receive the report from proxy 210 that device 240 is connected to client 102 and the device information. Agent 250 may further be configured to associate with the report from proxy 210 one or more identifiers for client 102 and/or for a user session through which client 102 is connected to server 104, such as a session number or a session locally unique identifier (LUID). Agent 250 can provide notification of device 240, along with the device information, to virtual bus driver 260. Virtual bus driver 260 (which may be a TCX USB bus driver, or any other bus driver) may be configured to create and store in memory a record corresponding to device 240, the record including at least part of the device information and session identifiers received from agent 250. Virtual bus driver 260 may be configured to report to operating system 170 of server 104 that device 240 is connected and to provide the device information to the operating system. This allows the operating system of server 104 to recognize the presence of device 240 even though device 240 is connected to client 102.
The operating system of server 104 may use the device information to find and load one or more appropriate device drivers for device 240 at server 104. Each driver may have an associated device object (object(s) 281a, 281b, . . . , 281n, referred to generally as device object(s) 281), as illustratively shown in FIG. 2. A device object 281 is a software implementation of a real device 240 or a virtualized (or conceptual) device 290. Different device objects 281 layer over each other to provide the complete functionality. The different device objects 281 are associated with different device drivers (driver(s) 282a, 282b, . . . 282n, referred to generally as device driver(s) 282). In an example, a device 240 such as a USB flash drive may have associated device objects including objects corresponding to a USB driver, a storage driver, a volume manager driver, and a file system driver for the device. The device objects 281 corresponding to a same device 240 form a layered device stack 280 for device 240. For example, for a USB device, a USB bus driver will create a device object 281a stating that a new device has been plugged in. Next, a plug-and-play (PNP) component of the operating system will search for and load the best driver for device 240, which will create another device object 281b that is layered over the previous device object 281a. The layering of device objects 281 will create device stack 280.
Device objects 281 may be stored in a memory of the server 104 associated with virtual bus driver 260. In particular, device objects 281 and resulting device stack 280 may be stored in random-access memory of server 104. Different devices 240/290 can have device stacks having different device objects and different numbers of device objects. The device stack may be ordered, such that lower level device objects (corresponding to lower level device drivers) have lower numbers than higher level device objects (corresponding to higher level device drivers). The device stack may be traversed downwards by traversing the stack from higher level objects to lower level objects. For example, in the case of an illustrative device stack 280 corresponding to a USB flash drive, the ordered device stack may be traversed downwards from a high-level file system driver device object, to a volume manager driver device object, to a storage driver device object, to a USB driver device object, and finally to a low-level virtual bus driver device object. Different device stacks 280 can be layered over each other to provide the functionality of the devices 240/290 inside devices, like USB Headsets, or USB pen drives. A USB pen drive, for example, can create a USB device stack first, over which it can create a storage device stack, where each of the device stacks have two or more device objects.
Once one or more device object(s) 281 are loaded by operating system 170 of server 104, each device object 281 can create a symbolic link (also referred to as a “device interface”) to device object 281 and associated device driver 282. The symbolic link is used by applications running on server 104 to access device object 281 and device 240/290. The symbolic link can be created by a call to a function such as IoCreateSymbolicLink( ) including such arguments as a name for the symbolic link, and a name of device object 281 or associated device 240. In one example, for example, a symbolic link to a USB flash drive device 240 is created by a call from a device object 281 for device 240 to the function IoCreateSymbolicLink( ) including arguments “\\GLOBAL??\C:” (i.e., the name for the symbolic link) and “\Device\HarddiskVolume1” (i.e., a name of the device object).
The creation of a symbolic link results in an entry being created in an object manager namespace (OMN) of operating system 170. The OMN stores information on symbolic links created for and used by operating system 170, including symbolic links for devices 240, virtualized devices 290, and applications 270 running on server 104.
As a result of the symbolic link creation process, a symbolic link to device 240 is enumerated in the OMN of server 104. Once the presence of device 240 is reported to operating system 170 of server 104, device 240 may be accessible from a user session (and associated desktop) running on server 104 (i.e., virtual desktop environment). For example, device 240 may appear as an icon on the virtual desktop environment and/or may be accessed by applications running on server 104.
An application 270 running on server 104 may access device 240 by sending a transaction request including the symbolic link for device 240 to operating system 170. Operating system 170 may consult the Object Manager Namespace to retrieve an address or other identifier for the device itself 240 or for a device object 281 associated with device 240. Using the retrieved address or identifier, operating system 170 forwards the transaction request for device 240 either directly, through a device object 281 of device stack 280, and/or through virtual bus driver 260. Virtual bus driver 260 may direct the transaction request to agent 250, which sends the transaction request to proxy 210 over network 106. Proxy 210 receives the transaction request from agent 250, and directs the received transaction request to stub driver 220. Stub driver 220 then directs the transaction request to device 240 through bus driver 230.
Bus driver 230 receives the result of the transaction request from device 240 and sends the result of the transaction request to stub driver 220. Stub driver 220 directs the result of the transaction request to proxy 210, which sends the result of the transaction request to agent 250 over network 106. Agent 250 directs the result of the transaction request to virtual bus driver 260. Virtual bus driver 260 then directs the result of the transaction request to application 270 either directly or through a device object 281 of device stack 280.
Thus, virtual bus driver 260 may receive transaction requests for device 240 from application 270 and send results of the transaction requests back to application 270 (either directly or through a device object 281 of device stack 280). As such, application 270 may interact with virtual bus driver 260 in the same way as with a bus driver for a device that is connected locally to server 104. Virtual bus driver 260 may hide the fact that it sends transaction requests to agent 250 and receives the results of the transaction requests from agent 250 instead of a device that is connected locally to server 104. As a result, device 240 connected to client 102 may appear to application 270 as if the physical device 240 is connected locally to server 104.
A key feature of USB devices is that all USB devices regardless of their device class employ the same connector. This allows any USB device to be coupled to any host in the same manner. The functionality that any USB device may provide is dictated by the USB controller chip within the USB device. For example, when coupled to a host, a controller chip of a USB keyboard can identify itself as a keyboard (or more specifically, as a device in the HID class) thereby causing the host to load appropriate drivers to enable the keyboard functionality on the host.
Recently, malware creators have exploited this USB functionality as a way to bypass malware scanners. Although malware scanners can typically scan the data region of a USB device (e.g., the storage region of a USB mass storage device), these scanners cannot access the USB device's firmware. Malware creators are therefore designing malware that will alter a USB device's firmware to reprogram it for a malicious purpose. For example, malware could be configured to modify a USB printer's firmware to emulate a keyboard (i.e., to cause the host to load the appropriate keyboard drivers). The firmware could also be modified to issue keyboard input (as if a user had actually typed on a keyboard) where the input would perform some malicious task such as deleting or corrupting files or installing malware on the host. The malware could in turn modify the firmware of other USB devices to enable the malware to spread to any other host to which the infected USB device may be connected. Similarly, the firmware could be modified to emulate a network card and change the host's DNS setting to redirect traffic. Many other types of malicious actions could also be taken. These types of exploits are commonly referred to as BadUSB.
As indicated above, there currently is no practical solution for preventing these exploits since malware scanners cannot access the firmware of a USB device. Also, recovery from these exploits is often incomplete. Even if a host is restored after a malware infection, because the source of the infection is in the USB device rather than the host itself, the host can quickly become re-infected if the USB device is again connected.